An apparatus for inflating an inflatable vehicle occupant restraint, such as an air bag, is disclosed in U.S. Pat. No. 5,184,846. The apparatus disclosed in the '846 patent includes a container which stores gas for inflating the vehicle occupant restraint. The apparatus also includes an ignition chamber which contains an ignitable propellant material. The propellant material, when ignited, produces combustion products including heat for augmenting the gas in the container. The ignition chamber is closed by a piston which blocks fluid communication between the propellant material in the ignition chamber and the gas in the container.
The apparatus disclosed in the '846 patent further includes an igniter for igniting the propellant material upon the occurrence of a vehicle collision. The resulting combustion products cause the gas pressure in the ignition chamber to increase to an elevated level. The gas pressure in the ignition chamber then moves the piston into an actuated position. When the piston moves into the actuated position, it opens the container to release the gas to flow from the container to the vehicle occupant restraint. The piston also opens fluid communication between the ignition chamber and the container. The combustion products in the ignition chamber are thus released to flow into the container to increase the pressure of the gas in the container.